An optimum switching or start/stop control is controlled by a stored program controller and is operative to control the turn-on and turn-off times of a ventilating heating or cooling system (sometimes referred to as HVAC) at the latest or earliest possible switching time, respectively, in order to achieve and maintain a desired comfort level during the occupancy of a building. This comfort level is allowed to degrade during unoccupied periods of the building when personnel comfort is not a factor in order to save energy.
Present optimum switching control systems tend to be expensive and many potential users do not implement these systems because of both high initial and high operating cost. Part of this initial high cost is the expense of the many analog temperature sensors plus the expense of analog-to-digital conversion units to couple a digitized version of the analog signal to the stored program controller. A further complication leading to the high operating cost is a tendency of the analog temperature sensing devices to drift over a period of time causing inefficiencies in the operation of the control system and further contributing to high maintenance cost.